Snowfoaming is worth doing each time you wash as the dwell time that this has on the surface of your car gives the chemicals time to loosen the debrisdirt that is on your paint.
when you rinse off, the car will be less dirty than when you started and importantly there is a reduced risk of you scratching/marring the paintwork as you have removed a lot of the contaminants.
A great new snowfoam that I am using is magifoam from
www.autobritedirect.co.uk - it has a long dwell time and is the nearest I have seen to a touchless wash - I PW'd both cars last weekend, snow foamed with Magifoam, left to dwell for 15-20 mins and rinsed off - they looked virtually spotless although I didn't risk drying the paintwork only the windows.